Sensitivity Analysis of Annual Nitrate Loads and the Corresponding Trends in the Lower Illinois River

Riverine nutrient concentrations and loads in the Midwestern United States have been identified as a major source of nutrients for the Mississippi River, causing hypoxia in the Gulf of Mexico. The Illinois River is one of the major contributors of nutrients to the Mississippi River, draining more than 40% of Illinois and parts of Indiana and Wisconsin. The main goals of this study were to calculate annual average nitrate concentration and annual total loads in the Lower Illinois River and its tributaries for 1975-2010, and to determine how the selection of different approaches affects the calculated trends. The Illinois Environmental Protection Agency (IEPA) has been collecting surface water quality data in Illinois since the early 1970s. This study used nitrite-N+nitrate-N (NO2+NO3) samples provided by IEPA and collected from the Illinois River at Havana and Valley City, Sangamon River at Oakford, and La Moine River at Ripley for the period 1975 to 2010. Discharge data concurrent with routine water quality samples were applied to a suite of approaches to estimate daily concentration and load for the period of analysis, including the seven-parameter regression equation along with several modeling residual adjustment techniques and the weighted regression on time, discharge, and season (WRTDS) method. The calculated annual mean concentration and annual total loads for NO2+NO3 for the period of record were discussed in relation to different approaches.